Chemical oxide removal of plasma damaged SiCOH low k dielectrics

ABSTRACT

A method for removing damages of a dual damascene structure after plasma etching is disclosed. The method comprises the use of sublimation processes to deposit reactive material onto the damaged regions and conditions to achieve a controlled removal of the damaged region. Furthermore a semiconductor structure comprising a dual damascene structure that has been treated by the method is disclosed.

FIELD OF THE INVENTION

The invention is related to a method of removal of damaged interleveldielectric or low-k material after trench etching prior to themetallization and during the dual damascene process.

BACKGROUND OF THE INVENTION

Damascene in semiconductor manufacture is the process of interconnectingmetal with the semiconductor device. These metal connections have to beestablished in a precise manner and interconnecting wires have to beisolated to avoid undesired interaction between the metal wires withother parts of the device.

In the early era of semiconductor manufacture, silica or silicateglasses were used as an insulator. As semiconductor devices becamefaster and thus smaller, metal connections on a device grew closerrequiring that the insulating material (silicates) become thinner.However, build up of charges in the thin layers of insulating materialcan result in an undesired “crosstalk” between metal connectors. Thischarge build-up and crosstalk could be avoided by the use of insulatingmaterial with a dielectric constant less than 3.9, which are referred toas low k dielectrics.

Chemically, silicates or silicon dioxide are structures where eachsilicon atom is surrounded by four oxygen atoms. Thereby, the negativecharges are predominantly located at the oxygen atoms and positivecharges are located at the silicon atom. These charge separationscontribute to the relative high dielectric constant of silica. A removalof oxygen in the silicate frame and replacement with groups such asalkyl radicals that do not induce a charge separation between thesilicon atom and the alkyl moiety have the result of a lower dielectricconstant. Another positive side effect is that these organic silicastructures, so called siloxanes are more porous, i.e., less dense thanits homologous silica and this contributes to an even lesser dielectricconstant. Therefore, some insulation materials used in damascence arelow-k Siloxanes, also referred to as SiLK or SiCOH materials.

Dual Damascence is a modified version of the damascene process. Here,prior to the metallization, the semiconductor structure is coated withSiCOH material. Then trenches for the metal connection are etched intothe structure using commonly adapted etching methods, such as plasmaetching. The etching process has a tendency to damage the SiCOH materialresulting in the introduction of oxygen into the SiCOH material, whichhas the effect of elevating the dielectric constant. The chemicalreaction can be described as:

In the past, the damage on the SiCOH material was removed using anaqueous dilution of hydrofluoric acid. However, dilute hydrofluoric acidalso etches the low-k material, thus resulting in a widening of thetrench. The aqueous solution also attacks other silicon containing partsof the device, e.g. masking layers such as TEOS oxides. Additionally,the penetration of the porous SiCOH material by the aqueous hydrofluoricacid solution may be intensified by capillary forces, thus resulting ina internal corrosion of the low-k material. Furthermore, the use ofaqueous solutions may impede the homogenous removal of damage across thesilicon wafer, since it is difficult to apply the solution uniformlyacross the surface. Additionally, in a solution based removal, thesurface tension of liquid droplets distributed onto the wafer may resultin a non-homogenous trim of the sidewalls of the newly formed trenches.

Accordingly, there exists a need in the art to overcome the deficienciesand limitations described hereinabove.

SUMMARY OF THE INVENTION

In a first aspect of the invention, a method for removing a plasmainduced damaged layer of low-k dielectric material includes thedeposition of a mixture that is generated from gaseous hydrofluoric acidand ammonia gas. The deposited mixture reacts with a surface comprisingthe damage and generates volatile reaction products. The method furtherincludes the removal of these reaction products.

In a second aspect of the invention, a method for trimming a dualdamascene structure of a semiconductor includes the deposition of amaterial generated from a dilute mixture of gaseous hydrofluoric acidand ammonia gas in a non-reactive diluent gas, followed by heating thestructure. The method further includes the flushing of the dualdamascene structure with a non-reactive flush gas.

In a third aspect, the invention includes a semiconductor device havinga plasma etched dual damascene structure with an interlevel dielectricfilm (ILD) whereby damage to a surface of the plasma etched dualdamascene structure is removed by depositing a controlled amount ofgaseous hydrofluoric acid and ammonia gas onto the surface of the dualdamascene structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d illustrate a method for removing damage from a surface ofa sidewall after a plasma etching process in accordance with theinvention; and

FIG. 2 depicts a graph showing variables for trimming damaged surfacematerial in accordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention relates to a method designed to remove damage that resultsfrom etching processes during the formation of a structure, and moreparticularly to the removal of damage that occurs to the interleveldielectric film (ILD) induced during plasma etching processes. Inembodiments, the method includes the deposition of a material generatedfrom gaseous hydrofluoric acid and ammonia gas onto the surface of theplasma etched dual damascene structures prior to metallization. Inembodiments, the gaseous mixture of hydrofluoric acid and ammonia gas isdiluted with a non-reactive diluent gas, such as noble gases (helium,neon, argon, xenon) or nitrogen.

FIG. 1 a represents a dual damascene structure prior to plasma etching,such as RIE. In General, the structure includes a wafer represented byreference numeral 10. A barrier cap or layer 12 comprising knownmaterial such as silicon nitride (Si₃N₄) is deposited on the siliconwafer 10 to any desired thickness. An interlevel dielectric film (ILD)14 is formed on the barrier layer 12. In embodiments, the ILD materialmay comprise silicon containing oxide material with organic groups thatresult in an insulating material with a decreased dielectric constant(low-k or SiCOH material). The ILD, in embodiments, comprises ofmaterial with a dielectric constant of 3.9 or less, and is preferablyorganic siloxanes.

A photoresist material 16 is formed and patterned on the layer 14, whichprotects the underlying material during the plasma etching process. Anycommonly known method for applying the photoresist can be used with theinvention. As should be understood, the etched trench can be about 100nm, but smaller and larger sizes are within the scope of the invention.

FIG. 1 b depicts the structure after the plasma etching process such asRIE. In this representation, a trench or via 17 is etched into thestructure, preferably to the silicon wafer 10. During the plasma etchingprocess, damage occurs to the sidewalls of the trench 17, whichcomprises low-k SiCOH material. This plasma-induced damage isrepresented by the hatched elements at reference numeral 18. Dependingon the nature of the low-k SICOH material, damage usually reachesbetween 1 to 25 nm into the low-k SiCOH material.

FIG. 1 c depicts the deposition of sublimed mixtures of gaseoushydrofluoric acid and ammonia gas onto the dual damascene structureafter plasma etching of the trench or via 17. The sublimed material isdepicted as reference numeral 20. In embodiments, the sublimation(formation of a precipitate from a gas) is carried out in a sublimationchamber and occurs by controlling deposition of both hydrofluoric acidand ammonia in the gas phase.

In embodiments, the material is deposited from a mixture of hydrofluoricacid and ammonia gas present in a stochiometric ratio from 4:1 to 1:4,preferably 2:1 (HF:NH₃); although, ratios outside these ranges arecontemplated. In further embodiments, a combined partial pressure of thegaseous hydrofluoric acid, the ammonia gas and a non-reactive diluentgas is between 1 and 20 millitorr, preferably 10 to 15 millitorr, withother ranges contemplated by the invention. In one preferred embodiment,a 10 millitorr pressure can be achieved by a combination ofapproximately 5 millitorr hydrofluoric acid, 2.5 millitorr ammonia and2.5 millitorr argon. In embodiments, the reduced pressure atmosphereeliminates surface tension of the HF as well as causes a retardedsublimation process to gain control of the amount of reactants beingsublimed onto the surface.

The method further comprises providing a flow of non-reactive diluentgas into the system, with flow ranges up to about 400 cm³/min, andpreferably between 0 and 250 cm³/min. (Other flow ranges are alsocontemplated by the invention.) It should be recognized that the flow ofthe non-reactive diluent gas increases the dilution of the gaseousmixture of the two reactive gases. Also, the dilution of the reactivegases results in a retardation of the deposition of the reactants,thereby permitting less material to sublime onto the surface thusallowing control of the amount of sublimed material and increasedcontrol of the removal of damaged material.

In an exemplary embodiment, as the deposited material forms on thedevice, a flow of argon enters the chamber at a rate up to about 250cm³/min. This contributes to the controllability of the method due to adilution of the reactive gases, ammonia and hydrofluoric acid. This mayresult in a slowdown or even cessation of the sublimation process.

In the embodiments of the invention, the hydrofluoric acid reacts withthe ILD 14 and the barrier layer 12 forming volatile reaction products,such as fluorosilanes and ammonium hydroxide. A completion of thereaction as well as transfer of the reaction products and the unreactedsublimed material into the gas phase can be achieved by heating thestructure to a critical temperature, between ambient and 200° C., andpreferably 150° C. The formed gaseous phase of the volatile reactionproducts and unreacted hydrofluoric acid and ammonia is carried awayfrom the chamber by either applying a vacuum or by flushing the systemwith a non-reactive flush gas such as, for example, argon or nitrogen.Flushing with a non-reactive flush gas may occur between 10 millitorrand atmospheric pressure, preferably 675 millitorr. In embodiments, avacuum step can follow the flushing of the structure.

As material 20 sublimes onto the structure, a chemical reaction betweenthe hydrofluoric acid and the damaged sidewall of SiCOH as well as theSi₃N₄ material occurs. Furthermore, the reactive sublimed material doesnot affect the elemental silicon of the wafer as well as thephotoresist. In further embodiments, the method also includes thatsublimation is carried out in the sublimation chamber but the reactioncan be delayed or carried out in another chamber.

As should be understood, in embodiments, implementation of the invention(e.g., ratio of reactive gas mixtures, diluent gases, pressure, flow,temperatures, and removal of reaction products) can be applied in amethod for trimming a dual damascene structure after plasma etching.That is, by using the method of the invention, a trench or via, withdamaged sidewalls, can be trimmed to clean the damage from the walls. Inthe processes described herein, the trimming can be controlled preciselyby the use of different pressures and temperatures; that is, forexample, an increased etch rate may be achieved by increasing thepressure conditions. Likewise, an increased etch rate may be achieved bydecreasing the diluent gas, for example, argon.

FIG. 1 d depicts the structure after the plasma induced damage has beenremoved. Previously undesired damage areas 18 are now removed and thelow-k SiCOH material 14 comprises a uniform sidewall.

FIG. 2 depicts a graph showing variables for trimming damaged surfacematerial in accordance with the invention. Generally, the graph showsargon flow vs. trim size. In this graph, three conditions andaccompanying results are shown: 10 mT, 50 HF/25 NH₃, 14 mT, 14 HF/7 NH₃,and a series three test. A non-lineal trendline connects the rhombusesto determine which argon flow would result in a desired trim size. Inthis representation, these conditions are applicable for desired trimsizes between approximately 6 and 25 nm. 10 mT, 50 HF/25 NH_(3.)

In this representation, the combined reduced pressure is 10 millitorr,partially distributed to 50% (5 millitorr) hydrofluoric acid (HF), 25%(2.5 millitorr) ammonia (NH₃), with the remainder being argon (25%, 2.5millitorr). Measure points depicted as rhombuses show the amount trimmed(x-axis) when these conditions are applied at various argon flowsfollowed by heating (to 150° C.) and flushing. For example, if theseconditions (10 millitorr, 50 HF/25 NH₃) are applied for two minutes atan argon flow of, for example, 25 cm³/min., the size trimmed from thesidewalls of the low-k material is about 10 nm. 14 mT, 14 HF/7 NH_(3.)

In this representation, the reduced pressure ranges at around 14millitorr combined from 14% (˜2.2 millitorr) hydrofluoric acid gas 7%(˜1.1 millitorr NH₃) and the remainder argon. Once generated in thereaction chamber, a trim size between 2 and 6 nm can be achieved byadjusting the flow of argon between about 40 cm³/min and 225 cm³/min.

While the invention has been described in terms of embodiments, thoseskilled in the art will recognize that the invention can be practicedwith the modification within the spirit and scope of the appendedclaims. For example, the invention can be readily applicable to bulksubstrates.

1. A method, comprising: depositing a mixture of gaseous hydrofluoricacid and ammonia gas onto a plasma induced damaged layer formed in atrench sidewall; forming volatile reaction products from a chemicalreaction between the plasma induced damaged layer and the depositedmixture of ammonia gas and gaseous hydrofluoric acid; and removing thevolatile reaction products with vacuum or a non-reactive flushing gas.2. The method according to claim 1, wherein a non-reactive diluent gasis used to dilute the mixture of gaseous hydrofluoric acid and ammoniagas.
 3. The method according to claim 2, wherein the non-reactive gasbelongs to a group of gases comprising nitrogen, helium, neon, argon. 4.The method according to claim 1, wherein the damaged layer is a low kdielectric material comprising organic siloxanes having a dielectricconstant of less than 3.9.
 5. The method according to claim 1, whereinthe mixture of gaseous hydrofluoric acid and ammonia gas has astochiometric ratio comprising a range from 4:1 to 1:4 (HF:NH₃).
 6. Themethod according to claim 2, wherein the mixture of gaseous hydrofluoricacid, ammonia gas and the non-reactive diluent gas has a combinedpartial pressure of 1 to 20 millitorr.
 7. The method according to claim2, wherein the non-reactive diluent gas is supplied at a flow between 0and 400 cm³/min.
 8. The method according to claim 1, wherein the formingof volatile reaction products is accelerated by adjusting thetemperature between ambient and 200° C.
 9. The method according to claim1, wherein the removing of the volatile reaction products includesflushing with nitrogen.
 10. The method according to claim 1, wherein theremoving of the volatile reaction products includes applying a vacuum.11. A method for trimming a dual damascene, comprising depositing amaterial generated from a dilute mixture of gaseous hydrofluoric acidand ammonia gas in a non-reactive diluent gas onto a trench of a dualdamascene structure; heating the dual damascene structure with thedeposited material; and flushing the dual damascene structure with anon-reactive flush gas to remove the material and chemically formedproducts.
 12. The method according to claim 11, wherein the trimming hasa thickness between 1 and 25 nm.
 13. The method according to claim 11,wherein the dilute mixture of gaseous hydrofluoric acid and ammonia gashas a stochiometric ratio between 4:1 and 1:4 (HF:NH₃), and thenon-reactive diluent gas is from a group comprising nitrogen, helium,neon, and argon.
 14. The method according to claim 13 wherein the dilutemixture of hydrofluoric acid and ammonia gas has a partial pressurebetween 1 and 20 millitorr, preferably between 10 and 14 millitorr andthe non-reactive diluent gas has a flow rate of 0 to 400 cm³/min,preferably 0 to 225 cm³/min.
 15. The method according to claim 11,wherein the heating of the dual damascene structure with the depositedmaterial occurs between ambient and 200° C., preferably 150° C.
 16. Themethod according to claim 11, wherein the non-reactive flush gas is oneof nitrogen, helium, neon, and argon.
 17. A method according to claim11, wherein the flushing of the dual damascene structure is followed byapplying a vacuum.
 18. The method according to claim 1, wherein theremoving of the volatile reaction products is subsequent to the formingof volatile reaction products.
 19. The method according to claim 11,wherein the flushing the dual damascene structure with a non-reactiveflush gas to remove the material and chemically formed products issubsequent to the heating the dual damascene structure with thedeposited material.
 20. The method according to claim 1, furthercomprising etching the trench through a photoresist material.
 21. Themethod according to claim 11, further comprising etching the trenchthrough a photoresist material.
 22. The method according to claim 1,wherein the depositing occurs through a photoresist material.
 23. Themethod according to claim 11, wherein the deposing occurs through aphotoresist material.
 24. The method according to claim 1, wherein theforming occurs while a photoresist material is arranged on a structurehaving the trench.
 25. The method according to claim 11, wherein theflushing occurs while a photoresist material is arranged on the dualdamascene structure.
 26. The method according to claim 1, furthercomprising forming the trench by etching into a dual damascene structurethrough a photoresist material.
 27. The method according to claim 1,further comprising forming the trench by etching into an interleveldielectric film (ILD) of a dual damascene structure.